Blended powder solid-supersolidus liquid phase sintering

ABSTRACT

A green article comprising an A-B powder mixture and methods of manufacturing such green articles and corresponding sintered articles are disclosed. The A-B powder mixture consists of a minor volume fraction of a relatively fine powder A and a complementary major volume fraction of a relatively coarse prealloyed powder B wherein the A-B powder mean particle size ratio is at least about 1:5. Metal powder A consists of one or more elemental metals or alloys which has a melting or solidus temperature above the highest sintering temperature at which the A-B powder mixture may be sintered without slumping. Prealloyed metal powder B consists of one or more alloys which are amenable to supersolidus liquid phase sintering. Green articles made from the A-B powder have a wider sintering temperature window than do articles made from prealloyed metal powder B alone.

STATEMENT AS TO RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.N00014-00-C-0378 awarded by the Office of Naval Research.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of powder metallurgy. Moreparticularly, the present invention relates to methods which involveblending a relatively fine metal powder with a relatively coarseprealloyed metal powder to produce a mixture that has a widenedsintering temperature window compared to that of the relatively coarseprealloyed metal powder. The present invention also relates to articlesmade from such powder mixtures.

2. Description of the Related Art

The science and industry of making and using powder metals is referredto as powder metallurgy. Powder metal compositions include elementalmetals as well as metal alloys and compounds. A wide variety ofprocesses are used to make powder metals, for example, chemical orelectrolytic precipitation, partial vaporization of metal containingcompounds, and the solidification of liquid metal droplets atomized frommolten metal streams. The shapes of metal powder particles areinfluenced by the powder making method and range from spherical toirregular shapes. Powder metals particles range in size from submicronto hundreds of microns. Particle size is measured as a diameter forspherical powders or as an effective diameter for non-spherical powders.

Various techniques are employed to consolidate powder metals particlesto form useful metal articles through the use of applied pressure and/orelevated temperatures. The powder metal to be consolidated is typicallyformed into a shape at room temperature and held in place through theuse at least one restraining mechanism such as container walls, afugitive binder material, or mechanical interlocking caused by pressingthe powder metal particles together with high pressure in a die press.Examples of specific forming processes include powder containerization,solid free-forming layer-wise buildup techniques (for example,three-dimensional printing (3DP) and selective laser sintering (SLS)),metal injection molding (MIM), and metal powder die pressing. The term“green article” is used herein to refer to the shaped powder metalarticle produced by this stage of the consolidation process. The greenarticle is then heated to one or more elevated temperatures at whichatomic diffusion and surface tension mechanisms become active toconsolidate the powder metal by sintering. The term “sintered article”is used herein to refer to the consolidated powder metal articleproduced by this stage of the consolidation process. Although sinteringmay occur to some extent over a range of temperatures as the greenarticle is being heated, the peak temperature to which the green articleis heated is what is usually referred to as the “sintering temperature.”Generally, the green article is held for a period of time ranging from afew minutes to a few hours at the sintering temperature, the length oftime depending upon a variety of process and metallurgicalsystem-related factors.

The heating of the green article is done in a controlled atmosphere orvacuum so as to protect the powder metal from undesired reactions withatmospheric constituents. The heating is also controlled so as toeliminate any fugitive binders from the green article. The consolidationof the green article into a sintered article is typically done at aboutatmospheric pressure or under vacuum. Some specialized techniques,however, such as hot isostatic pressing, hot uniaxial pressing, and hotextrusion, apply a pressure to the green article while it is hot to aidin the consolidation. In some processes, for example, in someembodiments of 3DP and SLS, consolidation is achieved through aninfiltration process by wicking a liquid metal into the pores of thegreen article from a source external to the green article.

As the consolidation of the powder metal proceeds from green article tosintered article, the density of the article increases as some or all ofits porosity is eliminated. Density, in this application, may be definedas “absolute density,” which is the article's mass per unit volume.Absolute density is expressed in terms such as grams per cubiccentimeter. Density is also defined as “relative density,” which is theratio of the absolute density of a powder metal article to the densitywhich that article would have if it contained no porosity. Relativedensity is expressed in terms of a percentage, with a highly porousarticle having a low relative density and an article having no porosityhaving a relative density of 100%. The relative density of a greenarticle depends on many factors and is sensitive to the method by whichthe green article was formed. Green article densities are generally inthe range of about 50-90%. The relative densities of sintered articlesalso depend on a variety of factors, including parameters of thesintering process. The sintered article relative densities typically arein the range of 75-95%. For applications in which the mechanicalstrength of a sintered article is of importance, high relative densitiesare generally desired. The increase in the relative density from thegreen article stage to the sintered article stage is referred to hereinas “densification.”

Densification may proceed by “solid state sintering,” which is a termthat describes the phenomena by which solid particles become joinedtogether at contact points through the diffusion of atoms between thecontacting particles. The number of point contacts for a given volume ofpowder and the ratio of surface area to particle volume increase asmetal powder particle size decreases, which results in finer powdermetals solid state sintering more readily and at lower temperatures thando larger powder metal particles.

Densification of a green article during sintering can be enhanced by thepresence of a liquid phase within the green article. The enhancementoccurs because of the relatively high atomic diffusion rates through aliquid as compared to a solid and because of the effect that the surfacetension of the liquid has in drawing the solid particles together. Thesintering that results from the presence of the liquid phase isidentified as “liquid phase sintering.” In some powder metallurgicalsystems, the powder metal in the green article comprises a minor volumefraction of a relatively low temperature melting powder metal and a highvolume fraction of a second type of powder metal which remains solid atthe sintering temperature. For example, a tungsten carbide-cobalt greenarticle may contain a low volume fraction cobalt powder, which is liquidat the sintering temperature, and a high volume fraction tungstencarbide powder, which remains solid at the sintering temperature.

An important variant of liquid phase sintering is supersolidus liquidphase sintering. Supersolidus liquid phase sintering is possible if aprealloyed powder metal passes into a solid-plus-liquid phase state uponheating. Referring to FIG. 1, which depicts a portion of an idealizedtemperature-composition equilibrium phase diagram 1 for an alloy systemconsisting of metal Y and metal Z, the horizontal axis 2 relates tocomposition with the left hand end 4 of horizontal axis 2 representingpure metal Y. The weight percentage of metal Z in the alloy compositionincreases linearly to the right along the horizontal axis 2. Thevertical axis 6 relates to temperature, which increases in the upwarddirection. The phase diagram 1 contains two phase boundary lines,liquidus line 8 and solidus line 10, which divide the illustratedportion of phase diagram 1 into three phase regions: a liquid phaseregion 12 above liquidus line 8; a solid-plus-liquid phase region 14between liquidus line 8 and solidus line 10; and a solid phase region 16below solidus line 10. A pure metal, such as metal Y, upon heating froma temperature at which it is a solid, remains a solid until it reachesits melting point temperature, T_(m) 18, at which it melts, and is aliquid at temperatures above T_(m) 18. In contrast, a Y-Z alloy ofcomposition X 20, upon heating from the solid phase region 16 alongdotted line 22 remains completely solid only until it crosses thesolidus line 10 at its solidus temperature T_(s) 24 and enters thesolid-plus-liquid phase region 14. In this region, the alloy exists inpart as a solid and in part as a liquid. Upon further heating, the solidfraction decreases and the liquid fraction increases until thetemperature crosses the liquidus line 8 at the alloy's liquidustemperature T₁ 26 into liquid phase region 12, wherein the alloy existsas all liquid. Upon cooling, the process is reversed, with the liquidtransforming upon crossing the liquidus line 8 into a liquid-plus-solidslush with an ever decreasing amount of liquid until the solidus line 10is crossed and the alloy once again becomes all solid.

“Supersolidus liquid phase sintering,” as used herein, refers to liquidphase sintering that occurs at a temperature that is between the solidustemperatures and the liquidus temperature of the particular alloycomposition. Supersolidus liquid phase sintering takes advantage of thetwo phase, solid-plus-liquid phase region 14 as a means of providing aliquid phase for liquid phase sintering. For example, if supersolidusliquid phase sintering is done at temperature T_(S) 28, a Y-Z alloy ofcomposition X 20 is in the solid-plus-liquid phase region 14 andconsists partly of a solid of composition X_(s) 30 and partly of aliquid of composition X₁ 32. The fraction of liquid present is equal tothe length of the sintering temperature dotted line 34 that is betweenits intersection point 38 with the solidus line 10 and its intersectionpoint 36 with the composition X dotted line 22 divided by the length ofthe sintering temperature dotted line 34 that is between itsintersection point 38 with the solidus line 10 and its intersectionpoint 40 with the liquidus line 8. A prealloyed metal powder is amenableto supersolidus liquid phase sintering if a sintering temperature existsfor the prealloyed metal powder at which the prealloyed metal powderdensifies by supersolidus liquid phase sintering without slumping.Slumping refers to a noticeable amount of gravity-induced distortion ofa green article occurring during liquid phase sintering that causes thedimensions of the resulting sintered article to be outside of theirrespective dimensional tolerance ranges. However, in micro-gravityconditions, slumping refers to such distortions which are surfacetension-induced, rather than gravity-induced.

The supersolidus liquid phase sintering of a powder metal is showndiagramatically in FIGS. 2A-C. Referring to FIG. 2A, a small portion 42of a green article is shown at great magnification before heating. Smallportion 42 includes powder metal particles 44 of a Y-Z alloycorresponding to the composition X 20 discussed above. It also includesa pore 46, which is indicated by the hatch area between the powder metalparticles 44. Powder metal particles 44 each contain grains 48.

Referring to FIG. 2B, the temperature of the small portion 42 of thegreen article has been raised to sintering temperature T_(S) 28 withinthe solid-plus-liquid phase region 14 of the phase diagram 1 that isshown in FIG. 1. A liquid phase 50 has formed between and around thegrains 48 of the powder metal particles 44. The liquid phase 50 hasbegun drawing the powder metal particles 44 closer together, shrinkingpore 46. Comparison with FIG. 2A shows that the grains 48 have changedshape as the liquid 50 forms and dissolution and reprecipitationprocesses occur.

Referring to FIG. 2C, the temperature of the small portion 42 of thegreen article is still at sintering temperature T_(S) 28, but sufficienttime has elapsed for the supersolidus liquid phase sintering to haveprogressed to the point where the pore 46 has been eliminated. Some ofthe grains 48 have become rearranged and changed in size and shape fromtheir initial state. Although the amount of liquid phase 50 is the sameas it was in FIG. 2B, it too has become redistributed. As a result ofthe sintering, the green article has densified.

The volume fraction of liquid phase that is present during any type ofliquid phase sintering, including supersolidus liquid phase sintering,is very important. An insufficient amount of liquid phase may beineffective in achieving the desired level of densification.Alternatively, an excess of liquid phase may result in slumping of thearticle. During liquid phase sintering, when most or all of theremaining solid grains or powder particles are surrounded by a liquidphase, the shape of the green article is maintained by surface tensionforces which give a high viscosity to the liquid-solid combination. Whenthe volume fraction of liquid phase is below a threshold level, thisviscosity is sufficiently high to permit the sintering green article toretain its shape at the sintering temperature long enough for thedensification to occur to a desired point. Above the threshold level,gravity-induced distortion exceeds a tolerable level before the desiredlevel of densification is achieved. The maximum amount of liquid phasethat can be tolerated during liquid phase sintering varies complexly andwidely and must be determined empirically.

Maintaining the article at the sintering temperature too long will alsoresult in slumping as gravity-induced viscous flow proceeds at a slow,but definite rate, even when the amount of liquid present at thesintering temperature is such that the viscosity is high. Additionally,time-related slumping may occur as coarsening of the solid phase grainsor particles causes a net decrease in their surface area that is incontact with the liquid and a corresponding increase in the thickness ofthe liquid layer between the grains, thereby decreasing viscosity.

Supersolidus liquid phase sintering is substantially affected by therate of change of the volume fraction of liquid phase with respect totemperature near the sintering temperature for a particular alloy. Ifthe volume of liquid phase increases rapidly with temperatures, thealloy is considered to be very sensitive to the sintering temperature.In some cases, the furnace temperature deviation about a set-pointtemperature, which in industrial furnaces can be on the order of tens ofdegrees Celsius, can exceed the temperature range in which a properrange of liquid phase fraction amounts are present in the alloy. Atemperature deviation to the low end of the temperature furnacetemperature deviation range could produce insufficient densificationwhereas a deviation to the high end of the temperature range couldresult in slumping. High precision temperature control usually comes atthe cost of lower through-put capacities and higher equipment prices.

Additionally, temperatures may vary from location to location within theworking zone a furnace. Among the relevant factors are a location'sproximity to heating elements, load and fixture related shielding fromradiative heating sources, and variations in gas flow patterns.Temperature variations during processing also occur within a greenarticle as the outside of the article heats up before its interior. Suchintra-article temperature variations are affected by the rate at whichthe furnace temperature is ramped up to the sintering temperature. Forexample, rapid ramping rates may cause large temperature differencesbetween the outside and inside of a green article, whereas more moderateramping rates allow time for better temperature equalization within thegreen article.

Any of the aforementioned process-related temperature variation factorsmay make it difficult or impracticable to sinter a particular greenarticle. In some cases, although supersolidus liquid phase sintering ofa particular green article is practicable, the processing-relatedtemperature variations make it necessary to use a lower sinteringtemperature within the solidus/liquidus temperature range and require acorrespondingly longer sintering time, in order to avoid slumping. Thisalso has the disadvantageous effect of lowering the throughput capacityof the furnace.

What is needed in the art is a process that will widen the window ofsintering temperatures within which a green article can be sintered toan acceptable density. Widening the sintering temperature window wouldmake the sintering of the green article less sensitive toprocess-related temperature variations. This could translate into theability to sinter the green articles in less expensive furnaces and athigher throughput rates.

SUMMARY OF THE INVENTION

The inventors have discovered that a green article comprising an A-Bpowder mixture may be sintered without slumping over a widenedtemperature range. Such an A-B powder mixture is made by mixing a minorvolume fraction of a relatively fine metal powder A, which has a meltingor solidus temperature that effectively exceeds the sinteringtemperature at which the powder mixture containing that powder issintered, with a complementary major volume fraction of a relativelycoarse prealloyed metal powder B, which is an alloy amenable tosupersolidus liquid phase sintering. A green article comprising the A-Bpowder mixture may be sintered without slumping into a solid article ata sintering temperature that is within a wider temperature range thancan a corresponding article which does not contain a volume fraction ofthe relatively fine metal powder A. “Relatively fine” and “relativelycoarse” signify that the mean particle sizes of the selected metalpowders A and B are related by a ratio of about 1:5 or higher, that is,that the mean particle size of metal powder B is at least about 5 timeslarger than the mean particle size of metal powder A. Metal powders Aand B may be of any shape.

The term “volume fraction” for a given powder refers to the portion ofthe occupied volume of a powder mixture which is actually occupied bythat particular powder. For example, in a powder mixture having a 100 ccoccupied volume of which 30 cc is occupied by powder A and 70 cc isoccupied by powder B, the volume fraction of powder A is 30% and thevolume fraction of powder B is 70%. The volume of any fugitive orreactive additives that may be added to a powder mixture of identifiedcomponents is not considered in determining the volume fraction of theidentified components. Thus, an addition of 5 cc of a fugitive additive,such as a polymer binder, or of a reactive additive, such as carbon, toan A-B powder mixture consisting of 30 cc of powder A and 70 cc ofpowder B does not influence the determination that the volume fractionof powder A in the mixture is 30% and the volume fraction of powder B is70%.

A green article “can be sintered without slumping” if no slumping occurswhen the green article is sintered to an achievable desired relativedensity in a reasonable time. Prolonged exposure to sinteringtemperatures for unreasonably long times can cause slumping due totime-related effects. Also, those skilled in the art will recognize thatthere is a limit to the relative density that is achievable by sinteringthat places an upper limit on the relative density that can reasonablybe achieved.

Green articles made from such A-B metal powder mixtures undergosubstantial densification to a given relative density at lower sinteringtemperatures than do corresponding green articles of relatively coarseprealloyed metal powder B which do not contain a volume fraction of therelatively fine metal powder A. Moreover, green articles of such powdermixtures may be sintered at measurably higher temperatures within thesolidus/liquidus temperature range of relatively coarse prealloyed metalpowder B without slumping than can corresponding green articles ofrelatively coarse prealloyed metal powder B that do not contain a volumefraction of the relatively fine metal powder A. Thus, the sinteringtemperature window or range of a green article is effectively widened,thus making it less susceptible to the deleterious effects oftemperature variations experienced in processing furnaces.

The relatively fine metal powder A particles occupy the intersticesbetween the relatively coarse prealloyed metal powder B particles in agreen article which comprises such an A-B powder mixture. Such particlepacking in the powder mixture increases the relative density of thegreen article. The relatively coarse prealloyed metal powder B isdifficult to sinter in the solid state, whereas the relatively finemetal powder A densifies relatively readily by solid state sintering.The benefit of using the fine powders is that, upon heating, solid statesintering of the relatively fine metal powder A densifies the greenarticle to a higher density at a lower temperature in comparison to acorresponding green article of only the relatively coarse prealloyedmetal powder B. Further increasing the sintering temperature above thesolidus temperature, but below the liquidus temperature, of therelatively coarse prealloyed metal powder B softens the relativelycoarse prealloyed metal powder B particles via a liquid phase that formswithin those particles, penetrating their grain boundaries. The liquidphase formation also causes the fast densification that is typical ofsupersolidus liquid phase sintering. The combination of the solid statesintering contributions of the relatively fine metal powder A and thesupersolidus liquid phase sintering contributions of the relativelycoarse prealloyed metal powder B results in the blended powder mixturesintering by what may be defined as solid-supersolidus liquid phasesintering. Moreover, the volume fraction of relatively fine metal powderA dimensionally stabilizes the green article during thissolid-supersolidus liquid phase sintering such that higher sinteringtemperatures can be employed without slumping in comparison to acorresponding green article of only relatively coarse prealloyed metalpowder B.

It is to be understood that the present invention is not limited tomethods and green articles comprising bimodal powder distributions.Higher-level poly-modal powder distributions are also contemplated, forexample, trimodal distributions. Such distributions contain a majorvolume fraction of a relatively coarse prealloyed metal powder B, whichis amenable to supersolidus liquid phase sintering, and a complementaryminor volume fraction of relatively fine metal A consisting ofsub-fractions such as sub-fractions A₁, A₂ and A₃. Each of therelatively fine metal powder sub-fractions A₁, A₂, and A₃ has a meltingtemperature or solidus temperature that exceeds the maximum sinteringtemperature at which the A-B powder mixture may be sintered withoutslumping. The mean particle sizes of metal powders A₁ and B are relatedby a ratio of about 1:5 or higher and the mean particle sizes of eachsuccessive pair-wise combinations of relatively fine metal powders, forexample, A₁ and A₂, A₂ and A₃, are related by a ratio of about 1:5 orhigher. These size ratios allow the finer powders to nest within theinterstices of the coarser powders.

It is also to be understood that in all cases, the relatively fine metalpowder A must remain essentially solid at the sintering temperature.“Essentially solid” identifies that, on the average, the relatively finemetal powder A particles must retain sufficient structural integrity andphysical size at the sintering temperature to act as physical barriersto the movement of the relatively coarse prealloyed metal powder Bparticles or grains thereof. Thus, some dissolution of metal powder Aparticles at or below the sintering temperature is permissible, as isthe formation of a small amount of internal liquid within metal powder Aparticles. It is also permissible for the metal powder A particles toreact with the metal powder B particles, even to form a small amount ofliquid, so long as the structural integrity and size criteria are met.In embodiments using higher-level poly-modal distributions, each of therelatively fine metal powder sub-fractions must be essentially solid atthe sintering temperature.

It is also to be understood that the relatively fine metal powder A mayconsist one or more elemental metals or alloys. For example, the powdervolume fraction identified as relatively fine metal powder A may be madeup a first volume sub-fraction of metal C and a second volumesub-fraction of metal D. In embodiments where higher-level poly-modaldistributions are employed, each of the relatively fine metal powderssub-fractions, for example A₁, A₂ and A₃, may consist of one or moreelemental metals or alloys which may be the same or different from thosein the other volume sub-fractions. The relatively coarse prealloyedpowder B may also consist of one or more alloys.

Green articles comprising a powder metal mixture having a minor volumefraction of a relatively fine metal powder A and a complementary majorvolume fraction of a relatively coarse prealloyed metal powder B arealso contemplated. In these embodiments, the relatively fine metalpowder A is an elemental metal or alloy whose melting temperature orsolidus temperature is higher than the highest sintering temperature atwhich the A-B powder mixture can be sintered without slumping and thecoarse prealloyed metal powder B is an alloy that is amenable tosupersolidus liquid phase sintering.

Methods of producing a green article having an enhanced sinteringtemperature range are also contemplated. Such a method includes thesteps of mixing together a minor volume fraction of a relatively finemetal powder A and complementary major volume fraction of a relativelycoarse prealloyed metal powder B to produce an A-B metal powder mixture,and forming a green article from said A-B metal powder mixture, whereinthe relatively fine metal powder A is a metal or alloy whose meltingtemperature or solidus temperature is higher than the highest sinteringtemperature at which the A-B powder mixture can be sintered withoutslumping, and wherein the coarse prealloyed metal powder B is an alloythat is amenable to supersolidus liquid phase sintering.

Methods of densifying a green article are also contemplated. Such amethod includes the steps of mixing together a minor volume fraction ofa relatively fine metal powder A and complementary major volume fractionof a relatively coarse prealloyed powder B to produce an A-B metalpowder mixture, forming a green article from said A-B metal powdermixture, and heating the green article to a sintering temperature belowthe liquidus temperature of the relatively coarse prealloyed metalpowder B to densify the green article by sintering, wherein therelatively fine metal powder A is an elemental metal or alloy whosemelting temperature or solidus temperature is higher than the highestsintering temperature at which the A-B powder mixture can be sinteredwithout slumping, and wherein the relatively coarse prealloyed metalpowder B is a metal alloy that is amenable to supersolidus liquid phasesintering. Where the relatively coarse prealloyed metal powder Bconsists of more than one alloy, the sintering temperature is atemperature that is lower than the liquidus temperature of each of thevarious relatively coarse prealloyed metal powder B alloys. In thismethod, the sintering temperature may, but need not, exceed the solidustemperature of the relatively coarse prealloyed metal powder B, or,where the relatively coarse prealloyed metal powder B consists of morethan one alloy, the sintering temperature may, but need not, exceed thesolidus temperature of any of those alloys.

Also contemplated are methods of solid-supersolidus liquid phasesintering a green article. Such a method comprises the steps of mixingtogether a minor volume fraction of a relatively fine metal powder A anda complementary major volume fraction of a relatively coarse prealloyedpowder B to produce an A-B metal powder mixture, forming a green articlefrom said A-B metal powder mixture, and heating the green article to asintering temperature between the solidus and liquidus temperatures ofthe relatively coarse prealloyed metal powder B, wherein the relativelyfine metal powder A is a metal or alloy whose melting temperature orsolidus temperature is higher than the sintering temperature at whichthe A-B powder mixture can be sintered without slumping, and wherein thecoarse prealloyed metal powder B is an alloy that is amenable tosupersolidus liquid phase sintering. Where the coarse prealloyed metalpowder B consists of more than one alloy, the sintering temperature is atemperature that exceeds the solidus temperature of each of the variousrelatively coarse prealloyed metal powder B alloys and is lower than theliquidus temperature of each of the various relatively coarse prealloyedmetal powder B alloys.

It is to be specifically understood that embodiments related tocorresponding green articles and methods involving higher-levelpoly-modal powder distributions are also contemplated.

Other features and advantages inherent in the subject matter disclosedand claimed will become apparent to those skilled in the art from thefollowing detailed description of presently preferred embodimentsthereof and to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The criticality of the features and merits of the present invention willbe better understood by reference to the attached drawings. It is to beunderstood, however, that the drawings are designed for the purpose ofillustration only and not as a definition of the limits of the presentinvention.

FIG. 1 is a portion of an idealized temperature-composition equilibriumphase diagram for a binary metal system Y-Z.

FIGS. 2A-C is a set of diagramatic drawings showing, at greatmagnification, the progressive densification of a small portion of aprior art green article of a powder metal alloy of composition X duringa prior art conventional supersolidus liquid phase sintering process.

FIG. 2A is a diagramatic drawing showing the small portion prior toheating.

FIG. 2B is a diagramatic drawing showing the small portion illustratedin FIG. 2A upon heating to a sintering temperature T_(S) that is betweenthe solidus and liquidus temperatures of composition X.

FIG. 2C is a diagramatic drawing showing the small portion illustratedin FIG. 2B at sintering temperature T_(S) after sufficient time haselapsed to eliminate the pore that was between the powder particles.

FIGS. 3A-B is a set of diagramatic drawings showing, at greatmagnification, the progressive densification of a small portion of agreen article embodiment of the present invention undergoingsolid-supersolidus liquid phase sintering wherein the coarse powder Bfraction of the green article is an alloy of composition X.

FIG. 3A is a diagramatic drawing showing the small portion prior toheating.

FIG. 3B is a diagramatic drawing of the small portion illustrated inFIG. 3A at solid-supersolidus liquid phase sintering temperature T_(SS)after the elapse of sufficient time for densification bysolid-supersolidus liquid phase sintering to have occurred.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In this section, some presently preferred embodiments of the presentinvention are described in detail sufficient for one skilled in the artto practice the present invention.

The metal powders used in the embodiments may be of any shape. However,the discussion is generally presented in terms of spherical powders,because spherical powders are easy to visualize and discuss. Also tosimplify the discussion, the terms “relatively fine metal powder A” and“relatively coarse prealloyed powder B” are shortened to “fine powder A”and “coarse powder B,” respectively, and sometimes just “powder A” and“powder B”, respectively, except where the expression of the full termaids in clarity.

The most basic powder mixtures in these embodiments consist of a minorvolume fraction of a relatively fine metal powder A and a complementarymajor volume fraction of a relatively coarse prealloyed metal powder B.These volume fractions may be adjusted to produce a mixture that has arelative density and a flowability which are optimized for the formingoperation in which the A-B powder mixture will be used to make a greenarticle. The highest relative density is achieved when the volumefraction of fine powder A is adjusted so that the interstices betweenthe coarse powder B particles are filled with fine powder A particles.However, fine powders do not flow as well as coarse powders. Thus, inoperations where high flowability is important, for example, in 3DP andSLS, it is preferred that the volume fraction of the fine powder A bebelow the level that would give the highest relative density. In otheroperations wherein flowability is not as critical, for example, in thedie pressing of simple shapes, the volume fractions may be adjusted togive the powder mixture that is close to or at the highest relativedensity.

Although powders of any size may be used in the embodiments contemplatedherein, in practice, it is very difficult to densify green articleshaving mean particle sizes over 160 microns. Also, because fine powdershave poor flowability and the selection of the mean particle size of therelatively fine metal powder A is to be related to mean particle size ofthe relatively coarse prealloyed metal powder B, it is preferred thatthe lower limit of the mean particle size of the relatively coarseprealloyed metal powder B be about 30 microns. More preferably, toprovide for good sinterability and flowability, the mean particle sizeof the powder B should be in the range of 45 to 60 microns. The particlesize may be measured by any conventional method from which a meanparticle size can be obtained, for example, laser diffractometry.

The mean particle size of the relatively fine metal powder A isdependent on the mean size of the metal powder B used. The ratio of themean particle size of fine powder A to that of the coarse powder B is tobe at least about 1:5 to provide for good filling of the intersticesbetween the coarse powder B particles with fine powder A particles. Morepreferably, the ratio is at least 1:7. Thus, when the mean diameter ofthe coarse powder B is 30 microns, the mean particle size of the finepowder A is no more than about 6 microns and is preferably less thanabout 4 microns. When the mean particle size of the coarse powder B is160 microns, the mean particle size of the fine powder A is no more thanabout 32 microns, and preferably less than about 23 microns. When themean particle size of the coarse powder B is 45 microns, the meanparticle size of the fine powder A is no more than about 9 microns andpreferably less than about 7 microns. Likewise, when the mean particlesize of the coarse powder B is 60 microns, the mean particle size of thefine powder A is no more than about 12 microns and preferably less thanabout 8 microns.

In more complex powder mixtures, a higher than binary poly-modal powderdistribution may be used. Such mixtures comprise a major volume fractionof a coarse powder B and a complementary minor volume fraction ofrelatively fine powder A consisting of two or more successively finersub-fractions, for example sub-fractions A₁, A₂, and A₃. The coarsepowder B has the same characteristics that the coarse powder B has withregard the simple binary distribution mixtures. Each of the fine powdersub-fractions has the same characteristics that fine powder A has withregard to simple binary distribution mixtures, except for the meanparticle size ratio relationship with powder B for all but the largestof the sub-fractions. In the case of higher-level poly-modal mixtures,the mean particle size of fine metal powder A₁ is related to that of thecoarse powder B by a ratio of at least 1:5 and more preferably at least1:7. The mean particle sizes of each successive pair-wise combination ofA₁ to A₂, A₂ to A₃, and so on, are related by a ratio of at least 1:5and more preferably at least 1:7. For example, for a trimodal powderdistribution wherein powder B has a mean particle size of 49 microns,and a 1:7 mean particle size ratio exists between powder B andsub-fraction powder A₁ as well as between sub-fraction powders A₁ andA₂, the mean particle sizes of powders A₁ and A₂ are, respectively, 7and 1 microns.

With regard to powder composition, the volume fraction of the powdermixture identified as the relatively fine powder A may be comprised ofone or more elemental metals or alloys. Examples of elemental metalsinclude iron, copper, and nickel. Examples of alloys include steels,stainless steels, nickel-based alloys, and copper-based alloys. Each ofthe elemental or alloys comprising powder A must be essentially solid atthe sintering temperature used for sintering the green article formedfrom the A-B powder mixture. In embodiments in which higher-levelpoly-modal distributions are employed, each of the sub-fractions may becomprised of one or more such elemental metals or alloys and the sameelemental metal or alloy may be in more than one sub-fraction.

The coarse powder B volume fraction, whether in A-B powder mixtureshaving bimodal or higher-level poly-modal distributions, may compriseone or more alloys. Each of the alloys comprising powder B must be analloy which is amenable to supersolidus liquid phase sintering. Examplesof such alloys include Inconel 625, Stellite, Haynes Alloy 613, andTribaloy T-800.

Fugitive or reactive additives may be added to the powder mixtures,either before or while the powder mixture is being formed into a greenarticle. The volumes of such additives are not taken into account in thedetermination of the volume fractions of the fine powder A and thecoarse powder B. Examples of fugitive additives include polymer bindersand lubricants such as wax, stearate, rubber, cellulose, and acrylics.Examples of reactive additives include carbon powder, boron powder, andother additives that are made in amounts of under 1 weight percent toadjust the composition of one or more of the components of the powdermixture during the consolidation process.

The powder mixtures may be prepared by conventional mixing or blendingtechniques, for example, by tumbling the fine powder A with the coarsepowder B in a V-blender or a double-cone blender. Preferably, the finepowder A and the coarse powder B are mixed or blended until a homogenousmixture is obtained. Localized distortion in the sintered article mayresult from over- or under-mixing. The powder mixture may also beprepared as a slurry by the incorporation of a volatile liquid, forexample, acetone or ethyl alcohol, either during or after the initialmixing of the A and B powders. Such a volatile liquid may be removed byevaporation either prior to or during the process of forming a greenarticle from the powder mixture.

Green articles may be prepared from the A-B powder mixtures by powdercontainerization, solid free-forming layer-wise buildup techniques,metal injection molding, or die pressing, using conventional procedures.Such green articles may be of any shape conventionally produced by suchforming processes ranging from very simple to very complex geometries.The preferable solid free-forming layer-wise buildup techniques that canbe used are 3DP and SLS.

The green articles may be sintered in any type of conventional sinteringfurnace using conventional heating rates and atmospheres. Where thegreen article contains a fugitive binder, conventional de-bindingconditions may be used to eliminate the fugitive binder.

The actual conditions to be used for heating and sintering the greenarticles is case-dependent, as required by the particular powdercompositions in the powder mixture and the powder fractions used for thefine powder A and the coarse powder B. Also relevant are factorspeculiar to the green article such as binder content and type, relativedensity, density gradients, size and geometrical complexity, as well asthe sintered relative density that it is desired for the sinteredarticle. “Sintered relative density” refers to the relative density ofthe sintered article. One skilled in the art will recognize that someexperimentation is required to determine the optimum heating andsintering conditions for any particular green article.

Sintering of the green articles may be done either above or below thesolidus temperature of the coarse powder B depending on the desiredsintered relative density. In general, sintering below the solidustemperature yields markedly lower sintered relative densities.Nonetheless, where the sintering temperature is below the powder Bsolidus temperature, the presence of the fine powder A in the greenarticle causes the green article to densify by solid state sintering toa higher sintered relative density than is achievable without the powderA fraction being present. Thus, the present invention reduces the lowtemperature end of the sintering window for the green article.

The green article may be sintered by solid-supersolidus liquid phasesintering. Referring to FIGS. 3A and 3B, the solid-supersolidus liquidphase sintering depicted there proceeds somewhat differently for greenarticle embodiments than it does for the case depicted in FIGS. 2A-C.Referring to FIG. 3A, a small portion 60 of a green article embodimentis shown at great magnification before heating. Small section 60includes fine powder A particles 62 and coarse powder B particles 64.Interparticle porosity 66 is indicated by the hatching in the areasbetween fine powder particles 62 and the coarse powder particles 64.Coarse powder B particles 64 are shown to each contain grains 68.

Referring to FIG. 3B, the temperature of small portion 60 of the greenarticle has been raised to the solid-supersolidus liquid phase sinteringtemperature T_(SS) and held there for sufficient time for densificationto occur by solid-supersolidus liquid phase sintering. Liquid phase 70has formed from the partial melting of the coarse powder B particles 64and has wicked into the interparticle areas and has drawn the coarsepowder B particles 64 closer together. Although all porosity that is inthe green article is not necessarily eliminated, the porosity 66 hasbeen eliminated from the small portion 60 and the green article hasdensified. The presence of the fine powder particles 62 in theinterstices between the coarse powder B particles 64, however, hasinterfered with the movement of the coarse powder B particle grains 68,thus stabilizing the overall structure of the green article andretarding the gravity-induced viscous flow that produces slumping, eventhough T_(SS) may be above the T_(S) used in the case that is shown inFIG. 2C.

Coarse powder B may comprise more than one alloy. In such cases, thegreen article may be densified by solid sintering alone by maintainingthe sintering temperature below the lowest solidus temperature of thepowder B component alloys. Solid-supersolidus liquid phase sintering maybe done at a temperature that is above the lowest solidus temperature ofthe powder B component alloys and below the lowest liquidus temperatureof the powder B component alloys. More preferably, thesolid-supersolidus liquid phase sintering is performed at a temperaturethat is within the solidus/liquidus temperature range of each of thepowder B component alloys so that each such alloy contributes liquid forthe densification of the green article.

The fine powder A also may comprise more than one metal or alloy. Eachpowder A constituent metal or alloy is essentially solid at thesintering temperature.

Following sintering, the sintered article may be subjected to additionalprocessing by conventional methods. For example, the sintered articlemay be hot isostatically pressed to achieve further densification.

EXAMPLES

The following non-limiting examples are given for further illustrationof the present invention, but are not intended to limit its scope atall.

Example 1

A-B powder mixtures were prepared using pure copper powder (vendor gradedesignation, 2000; 99.0% purity) as the relatively fine metal powder Aand a bronze alloy (vendor grade designation, 40P) as the relativelycoarse powder B. The composition of the bronze alloy was, in weightpercent: 9.23% tin, 0.0496% phosphorus, balance copper. The copper andthe bronze powders were spherical powders obtained from AcuPowderInternational, LLC, of Union, N.J., U.S. The copper powder had a meanparticle size of about 5 microns and the bronze powder had a meanparticle size of about 118 microns as measured on a laser diffractometryparticle size analyzer. Thus, the A-B powder particle size ratio was23.6. Differential scanning calorimetry measurements showed the solidustemperature of the bronze powder to be 850° C. and its liquidustemperature to be 1016° C. The melting point of the copper powder wastaken to be that of pure copper, that is, about 1083° C., which exceedsthe liquidus temperature of the bronze powder by 67° C.

Open-top containerized green samples were prepared from a dozendifferent powder mixtures in which volume fractions of the copper andbronze powders were varied as shown in Table 1.

Each sample was prepared by weighing out the copper and bronze powderson a laboratory balance to yield the desired volume fraction levels. Thepowders were blended together for 30 minutes in a Turbula® blender. Thepowder mixture was containerized by pouring it into a 20 ml aluminacrucible and tapping it 20 strikes. The open containerized powdersamples constituted the green articles in this example. The greenarticle absolute density was measured by the mass-divided-by-volumemethod and the green relative density was calculated therefrom. Thesevalues appear in Table 1 for each sample. The green relative densityincreased as the volume fraction of the copper powder increased untilthe green relative density reached a peak at 73.2% for a copper powdervolume fraction of 31% and decreased thereafter as the volume fractionof the copper volume fraction increased. The data shows that the greensamples made from blends 2-6 had green relative densities that weresignificantly higher than that of the prior art blend 1 green articlewhich was made from the bronze powder that had no copper powderadditions.

The samples were placed in a tube furnace and heated at a rate of 5° C.per minute in an atmosphere of 80% nitrogen and 20% hydrogen to asintering temperature, held at the sintering temperature for 30 minutes,and then cooled to room temperature. The sintered article absolutedensity of each sample was measured using the Archimedes method and thesintered article relative densities were calculated therefrom. Linearshrinkage was calculated for the samples after sintering based uponradial dimension measurements.

Solid state sintering below the bronze powder solidus temperature wasconducted at 700° C. The data in Table 1 shows that the relative densityof the sintered article generally increased as the volume fraction ofthe fine copper powder increased. The data in Table 2 shows that thelinear shrinkage for the sintered samples generally increased as thevolume fraction of the fine copper powder increased. This indicates thatthe addition of a volume fraction of fine copper powder increased theamount of solid state sintering.

Sintering of similarly prepared samples was done at 920° C. The data inTable 1 shows that the relative density of the sintered samples againgenerally increased as the volume fraction of the fine copper powderincreased. The data in Table 2 shows that the linear shrinkage for thesamples initially decreased as the volume fraction of the fine copperpowder increased until a minimum of about 7.6% was reached at a 20% finecopper powder volume fraction. At higher volume fractions of the finecopper powder, the shrinkage generally increased as the volume fractionof the fine copper powder increased. Comparing this to the solid statesintering behavior, the unexpected result obtained is that the additionof a minor volume fraction of relatively fine copper powder to a majorvolume fraction of a relatively coarse bronze powder dimensionallystabilized the green article during super-supersolidus liquid phasesintering while providing an improvement in densification.

TABLE 1 Supersolidus Supersolidus Solid Solid Liquid Liquid State StatePhase Phase Sintered Sintered Sintered Sintered Volume Volume GreenGreen Article Article Article Article Fraction Fraction Article Article(700° C.) (700° C.) (920° C.) (920° C.) Fine Coarse Absolute RelativeAbsolute Relative Absolute Relative Blend Powder Powder Density DensityDensity Density Density Density No. (Copper) (Bronze) (g/cc) (%) (g/cc)(%) (g/cc) (%) 1 0 100 5.282 59.3 5.491 61.6 7.753 87.0 2 10 90 5.67663.7 5.955 66.8 7.906 88.7 3 20 80 6.117 68.6 6.629 74.3 8.034 90.1 4 3070 6.503 72.9 6.950 77.9 8.179 91.6 5 31 69 6.529 73.2 6.880 77.1 8.25992.5 6 40 60 6.303 70.6 7.043 78.9 8.188 91.7 7 50 50 5.908 66.1 7.15880.1 8.223 92.0 8 60 40 5.691 63.7 7.098 79.4 8.236 92.1 9 70 30 5.37460.1 7.185 80.3 8.305 92.8 10 80 20 5.203 58.1 7.208 80.5 8.391 93.8 1190 10 4.843 54.1 7.244 80.9 8.464 94.5 12 100 0 4.717 52.6 7.420 82.88.311 92.8

TABLE 2 Supersolidus Solid Liquid State Phase Volume Volume SinteredSintered Fraction Fraction Article Article Fine Coarse (700° C.) (920°C.) Blend Powder Powder Shrinkage Shrinkage No. (Copper) (Bronze) (%)(%) 1 0 100 0.9 10.9 2 10 90 1.3 8.9 3 20 80 2.7 7.6 4 30 70 4.2 8.4 531 69 4.1 9.0 6 40 60 5.0 9.0 7 50 50 6.2 10.1 8 60 40 6.7 11.1 9 70 308.5 13.3 10 80 20 9.9 15.4 11 90 10 11.8 16.4 12 100 0 15.2 18.0

Example 2

In Example 1, it was determined that a powder mixture of 31 volumepercent copper powder and 69 volume percent bronze powder (blend 5)yielded the highest green relative density of the blends tested. In thepresent example, additional green article samples were prepared usingthis blend 5 powder mixture and using the bronze powder (blend 1) in themanner described in Example 1. The samples were sintered at temperaturesranging from 550 to 960° C. to determine sintering temperaturesensitivity. Density measurements were made on the samples aftersintering and are tabulated in Table 3.

The results show that the copper-bronze powder mixture samples sinteredto a higher density at each sintering temperature than did the bronzepowder samples. This demonstrates that a desired relative density can beachieved at a lower temperature with the blend 5 samples. For example,while a sintering temperature of 930° C. was required to achieve arelative density of 80% for a bronze powder sample, this density wasachieved by sintering the copper-bronze powder mixture at only 860° C.

The results show that the copper-bronze powder mixture samples could besolid-supersolidus liquid phase sintered without slumping at atemperature that was 20° C. above the temperature at which slumpingoccurred in the bronze-only powder samples. Thus, the exampleillustrates the widening of the sintering temperature window bypermitting a green article containing a fine metal powder/coarseprealloyed metal powder mixture to be sintered to a desirable relativedensity without slumping at temperatures that are both higher and lowerthan those that can be used for sintering a green article prepared onlywith the coarse prealloyed metal powder.

TABLE 3 31 Copper/69 Bronze Bronze (Blend 1) (Blend 5) Sintered SinteredSintering Relative Relative Sintering Mode for Temperature DensityDensity Sintering Mode for 31 Copper/69 Bronze (° C.) (%) (%) BronzePowder (Blend 1) Powder (Blend 5) 550 59.5 70.4 Solid state Solid state600 59.8 75.6 Solid state Solid state 650 60.2 77.9 Solid state Solidstate 700 60.6 78.9 Solid state Solid state 750 60.5 79.2 Solid stateSolid state 800 60.7 79.4 Solid state Solid state 850 62.1 79.5Supersolidus Solid-supersolidus liquid phase liquid phase 860 63.7 80.1Supersolidus Solid-supersolidus liquid phase liquid phase 870 65.8 81.0Supersolidus Solid-supersolidus liquid phase liquid phase 880 68.5 82.3Supersolidus Solid-supersolidus liquid phase liquid phase 890 71.6 84.4Supersolidus Solid-supersolidus liquid phase liquid phase 900 74.6 86.5Supersolidus Solid-supersolidus liquid phase liquid phase 910 77.1 87.6Supersolidus Solid-supersolidus liquid phase liquid phase 920 79.6 88.4Supersolidus Solid-supersolidus liquid phase liquid phase 930 80.1 89.5Supersolidus Solid-supersolidus liquid phase liquid phase 940 Slumped90.2 Supersolidus Solid-supersolidus liquid phase liquid phase 950Slumped 92.8 Supersolidus Solid-supersolidus liquid phase liquid phase960 Slumped Slumped Supersolidus Solid-supersolidus liquid phase liquidphase

Example 3

A-B powder mixtures were prepared using commercially pure nickel powder(carbonyl-derived nickel powder) as the relatively fine metal powder Aand a nickel alloy (vendor grade designation, Superbond 625) as therelatively coarse prealloyed metal powder B. The nickel alloy powder wasspherical and had a composition, in weight percent, of 21% chromium, 9%molybdenum, 4% columbium, 0.1% carbon, and balance nickel and wasobtained from ANVAL Inc., of Rutherford, N.J., U.S. The pure nickelalloy was of spherical particle shape and was obtained from ChemalloyCompany, Inc., of Bryn Mawr, Pa., U.S. The pure nickel powder had a meanparticle size of about 10 microns and the nickel alloy powder had a meanparticle size of about 79 microns as measured on a laser diffractometryparticle size analyzer. Thus, the A-B powder particle size ratio wasabout 1:8. Differential scanning calorimetry measurements showed thesolidus temperature of the nickel alloy powder to be 1270° C. and theliquidus temperature to be 1368° C. The melting point of the pure nickelpowder was taken to be that of pure nickel, that is, about 1453° C.,which exceeds the liquidus temperature of the bronze powder by 85° C.

Samples made from a blended mixture having 17.3% volume fraction of thepure nickel powder and 82.7% volume fraction of the nickel alloy powderwere prepared in the same manner as was described in Example 1. Samplescontaining only the pure nickel alloy powder were likewise prepared. Thegreen relative density of the blended mixture samples was 62.3% whereasthat of the nickel alloy powder samples was only 58.6%.

The samples were sintered in a tube furnace. They were heated at a rateof 5° C. per minute in an atmospheric mixture of argon and hydrogen to asintering temperature, held at the sintering temperature for 30 minutes,and then cooled to room temperature. The sintering temperatures were inthe range of from 1216 to 1365° C. The sintered article absolute densityof each sample was measured using the Archimedes method and the sinteredarticle relative densities were calculated therefrom. The data ispresented in Table 4.

The results show that both types of samples underwent solid statesintering at temperatures below the 1270° C. solidus temperature of thenickel alloy powder. However, the relative densities of the blendedmixture samples were significantly higher than those of the nickel alloypowder samples.

Both types of samples showed greater densification when sintered attemperatures above 1270° C. The rate of increase in densification withtemperature was much greater for the nickel alloy powder samples. Thus,even though the sintered relative density of the nickel alloy powdersample at 67.9% was significantly below the 74.0% of the blended mixturesample at the 1272° C. sintering temperature, the sintered relativedensities for the 1315° C. sintering temperature of the two types ofsamples was nearly equal at about 80.5% for the nickel alloy powdersample and 80.9% for the blended mixture sample. By the 1329° C.sintering temperature, the nickel alloy powder sample sintered relativedensity had jumped up to 92.2% and by the 1338° C. sinteringtemperature, the nickel alloy powder sample had slumped. This indicatesthat the nickel alloy powder has a great sensitivity to the sinteringtemperature, making tight control of the furnace sintering temperaturevery critical. In contrast, the data illustrates a much lowersensitivity to the sintering temperature for the blended mixturesamples. The increase in densification progressed with increasingtemperature at a much slower rate than it did with the nickel alloypowder samples such that a sintered relative density of 92.5% was notreached until the sintering temperature was raised to 1356° C. Thus, theexample illustrates that the blended powder mixture is much lesssensitive to the sintering temperature.

Moreover, it was not until the 1365° C. sintering temperature wasreached that slumping occurred for the blended mixture sample. Thus, theexample demonstrates that the blend 5 samples were able to be sinteredwithout slumping over a wider range of temperatures than could be thenickel alloy powder samples which did not contain a volume fraction ofthe relatively fine pure nickel powder.

TABLE 4 17.3 Nickel/ Nickel 82.7 Alloy Nickel (625) Alloy SinteredSintered Sintering Relative Relative Sintering Mode Sintering Mode for17.3 Temperature Density Density for Nickel Alloy (625) Nickel/ (° C.)(%) (%) Powder 82.7 Nickel Alloy Powder 1216 62.4 72.5 Solid state Solidstate 1235 63.0 72.7 Solid state Solid state 1254 64.6 72.9 Solid stateSolid state 1272 67.9 74.0 Supersolidus liquid Solid-supersolidus phaseliquid phase 1301 74.7 78.4 Supersolidus liquid Solid-supersolidus phaseliquid phase 1315 80.5 80.9 Supersolidus liquid Solid-supersolidus phaseliquid phase 1329 92.2 85.2 Supersolidus liquid Solid-supersolidus phaseliquid phase 1338 Slumped 87.8 Supersolidus liquid Solid-supersolidusphase liquid phase 1347 Slumped 90.3 Supersolidus liquidSolid-supersolidus phase liquid phase 1356 — 92.5 Supersolidus liquidSolid-supersolidus phase liquid phase 1365 — Slumped Supersolidus liquidSolid-supersolidus phase liquid phase

Example 4

In this example, a 700 gram sintered article of a complex shape wasproduced. The complex shape was a valve having convoluted internalpassageways. The green article precursor of the sintered valve was madeby the 3DP solid free-forming layer-wise buildup technique using ablended powder mixture similar to that described in Example 3, exceptthat the mean particle size of the nickel powder was about 8 microns.The powder mixture was prepared in the manner described in Example 3.

The 3DP (Three Dimensional Printing) process allows a green article tobe made directly from a computer model without the need for molds ordies. The process creates a green article by a layer-wise printingprocess in which a computer controlled print-head selectively deposits afugitive binder onto each new powder layer that is applied to a powderbed supported on a vertically-indexable piston. The information forcontrolling the print-head is obtained by applying a slicing algorithmto the computer model. Powder particles in portions of each newlyapplied powder layer are selectively joined to each other and toparticles in the previous layer by the “ink-jet”-like printing of thebinder material from the print-head onto the powder layer. The powderbed is indexed downward after it has been selectively scanned by theprint head and the next sequential layer is applied and selectivelyprinted with the binder material. The process is repeated until theentire shape of the green article is formed. At the end of the process,the green article is separated from the rest of the powder bed, forexample, by pouring off the loose powder.

In this example, the 3DP system that was used to make the green articlewas a RTP 300 unit manufactured by Extrude Hone Corporation of Irwin,Pa., U.S. An acrylic polymer binder was used. The relative density ofthe green part was determined to be 61.6%.

The green article was placed in an atmosphere furnace. Binder removaland presintering was accomplished by holding the green article first at500° C. for 30 minutes and then at 950° C. for 30 minutes under aprocess atmosphere consisting of 95% argon and 5% hydrogen. The greenarticle was then heated in a vacuum furnace at a rate of 5° C. perminute to a sintering temperature of 1335° C. and held for 45 minutes atthat temperature, and then cooled to room temperature. Note that the1335° C. sintering temperature employed is approximately the sametemperature at which slumping occurred in the nickel alloy powder samplein Example 3, as can be seen by reference to Table 3. Nonetheless, thesintered article was found to have not undergone slumping. The relativedensity of the sintered article was determined to be about 92.5%.

This example was repeated an additional three times with nearlyidentical results each time.

A significant advantage illustrated by this Example is thatdensification was achieved without the use of the infiltrating operationthat is usually required in conventional solid free-forming layer-wisebuildup techniques. In such infiltration operations, a secondary liquidmetal is wicked into the pores of the green article from a sourceexternal to the green article. The elimination of the infiltrationoperation reduces total process cost by reducing set-up processing andtime and furnace time. It also avoids the technical difficulties andrejection costs associated with infiltrant erosion of the greenarticle's printed skeleton. Moreover, the material properties of thefinal article do not need to be compromised by the presence aninfiltrant as an article comprising just the constituent powders is madepossible with the practice of embodiments of the present invention.

Although the instant example was conducted using the 3DP method,embodiments of the present invention utilizing solid free-forminglayer-wise buildup techniques are not limited thereto, but ratherinclude all known solid free-forming layer-wise buildup techniques whichare compatible with the use of metal powder as a skeleton building blockparticle. Thus, embodiments include, without limitation, techniques inwhich light-activated photopolymer powders or liquids are mixed into,with or without a volatile liquid, or sprayed onto skeleton buildingblock particles which are then layer-wise deposited in dry, spray, orslurry form and, possibly after an intermediary step of removing avolatile liquid, are subsequently cemented in place as part of the greenarticle's printed skeleton by the selective application of light, forexample, from a scanning laser or through a light-wavelength filteringmask, whether or not those techniques have heretofore utilized metalpowders as skeleton building block particles. The green articlesproduced by these techniques may then be heated under suitableatmospheres or vacuum to remove the fugitive binders and then sinteredto produce a densified article.

While only a few embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art that manychanges and modifications may be made thereunto without departing fromthe spirit and scope of the invention as described in the followingclaims.

What is claimed is:
 1. A green article comprising a powder metal mixturehaving a minor volume fraction of a relatively fine metal powder and acomplementary major volume fraction of a relatively coarse prealloyedmetal powder, wherein the relatively fine metal powder is essentiallysolid at the highest sintering temperature at which the green articlecan be sintered without slumping, the relatively coarse prealloyed metalpowder is amenable to supersolidus liquid phase sintering, and the ratioof the mean particle size of the relatively fine metal powder to themean particle size of the relatively coarse prealloyed metal powder isat least about 1:5.
 2. The green article of claim 1, wherein therelatively fine metal powder consists of two or more elemental metals ormetal alloys, each of said elemental metals or metal alloys beingessentially solid at the highest sintering temperature at which thegreen article can be sintered without slumping.
 3. The green article ofclaim 1, wherein the relatively coarse prealloyed metal powder consistsof two or more metal alloys, each of said metal alloys being amenable tosupersolidus liquid phase sintering.
 4. A green article comprising apowder metal mixture having a minor volume fraction of a relatively finemetal powder and a complementary major volume fraction of a relativelycoarse prealloyed metal powder, wherein said minor volume fraction of arelatively fine metal powder consists of at least two sub-fractions ofsuccessively finer mean particle size, each of said successivesub-fractions consists of metal powder that is essentially solid at thehighest sintering temperature at which the green article can be sinteredwithout slumping, the relatively coarse prealloyed metal powder isamenable to supersolidus liquid phase sintering, the ratio of the meanparticle size of the coarsest of said successive sub-fractions to themean particle size of the relatively coarse prealloyed metal powder isat least about 1:5, and the ratio of the mean particle size of eachfiner successive sub-fraction to the mean particle size of animmediately preceding coarser successive sub-fraction is at least about1:5.
 5. The green article of claim 4, wherein at least one of thesuccessive sub-fractions consists of two or more elemental metals ormetal alloys, each of said elemental metals or metal alloys beingessentially solid at the highest sintering temperature at which thegreen article can be sintered without slumping.
 6. The green article ofclaim 4, wherein the relatively coarse prealloyed metal powder consistsof two or more metal alloys, each of said metal alloys being amenable tosupersolidus liquid phase sintering.